Protective coatings are vital to the continued performance and life of industrial and marine gas turbines, the hot section components of which are subjected to hostile enivornments at temperatures between 1300.degree. F. and 1800.degree. F. Because blade and vane alloy compositions meeting mechanical property requirements do not exhibit acceptable sulfidation/oxidation resistance for sustained operation in marine and industrial gas turbines, it is necessary to provide protective coatings which are metallurgically stable adn compatible with the substrate alloy and do not significantly degrade its mechanical properties at operating temperatures.
Aluminum, silicon and chromium are the only three alloying elments which form self-healing protective oxide surface layers oon nickel-, cobalt- and iron-base superalloys. Early prior art includes aluminide coatings which are more protective at higher temperatures and chromium and silicon coatings which perform better at the lower end of the temperature spectrum experienced by gas turbine hot sections. Also included in prior art are the MCrAlY class of coatings where M represents iron, cobalt, nickel or certain combinations thereof. In some service environments, MCrAlY coatings have demonstrated an advantage over aluminide coatings relative to corrosion resistance and ductility. All heretofore known coatings for superalloy blades/buckets, however, have deficiencies that limit their usefulness. The long-sought goal for coating developers has been to eliminate those deficiencies and to broaden the protective temperature range.